The problems with higher transmit power on the router is that a) it increases noise and b) it does nothing for the return signal coming _from_ the client. Higher gain antennas are nearly *always* the correct solution, not increasing power on one side of the connection.

a) You don't have to reboot, the changes take effect right away.
b) It won't melt. Why would it? The chipset was designed to run at those powers. However, the "mW" label in Tomato probably doesn't mean very much. Best to regard it as an arbitrary, relative number.
c) Rodney is correct in that a better antenna might increase signal strength of both transmit and receive. However, an antenna with higher "gain" only increases signal in one direction at the expense of others, so it may make your problem worse in some areas.
d) We are usually mostly concerned with high download speeds. We typically send much less in the upload direction, so it doesn't have to be as fast as our downlink.
e) Tomato and all wifi radios negotiate link speeds with clients. So your client will always negotiate a reliable uplink speed with your router. It doesn't matter how slow it is as long as it is reliable, even 1Mbps would be adequate for most needs.
f) Therefore, if you increase your router's transmit power, making the link "assymmetrical" - your client will now negotiate a faster downlink (receive) speed, leaving the uplink (transmit) speed unchanged.
g) By doing this, downloads to your router will be improved, the uploads will be unchanged. As pointed out above, our upload speed does not have to be as fast, so there is nothing wrong with doing this.
h) The power amplifier chipsets are generally designed to be linear and unconditionally stable up to the rated power output. If not, then they couldn't pass FCC type approval. I've worked in one of these approvals labs, and I can assure everyone that the things don't suddenly radiate crap everywhere if you increase the power a tad.
i) Of course, they may interfere with other systems more if they are co-located on the same channel, and you must choose between channels 1,6, and 11. Running on channels other than these can really cause problems if you are in an area with many routers active. Google for information on why.
i) Typically, in a steel rebar reinforced concrete building, increasing transmit power dramatically improves client's "happiness" rating when their download speeds increase from a few hundred kbps to several Mbps. This can happen when you are just on the "threshold" and a little more signal strength lifts you out of this zone.
j) If you are already close to your router, increasing the transmit power will achieve nothing much except perhaps overloading the radios and making things worse.

Thanks anyone, I tried different values (50, 100, 150) and each time I walked around the house with inSSIDer running on my netbook, I can not see difference really… the dB value varies +/-3 depending of where I am in the house but nothing concistent. Even in a different floor I can see 300MBps in the active list on Tomato web page.

I let it at 0 and live with it :) but I still want to test 400 :)

Don't fly in anything with a Capissen 38 engine, they fall right out of the sky…

To diagnose, in "wireless settings" turn off all of these additional options that may be causing problems and see if your problem goes away. Of particular note is the "interference mitigation" setting which is responsible for a lot of problems.

TX power doesn't even work in Tomato USB/RAF and anything really, with the E3000 at least. 71mW to 251mW does absolutely nothing with router signal strength, range, or throughput. With DD-WRT, 71mW to 251mW does a large increase.